In conjunction with the recent rapid advances of portable electronic equipment and communications instruments, nonaqueous electrolyte secondary batteries having a high energy density are strongly demanded from the aspects of cost, size and weight reductions. Approaches known in the art to increase the capacity of such nonaqueous electrolyte secondary batteries include, for example, use as negative electrode material of oxides of B, Ti, V, Mn, Co, Fe, Ni, Cr, Nb, and Mo and composite oxides thereof (JP 3008228 and JP 3242751); application as negative electrode material of M100-xSix wherein x≧50 at % and M=Ni, Fe, Co or Mn which is obtained by quenching from the melt (JP 3846661); use as negative electrode material of silicon oxide (JP 2997741); and use as negative electrode material of Si2N2O, Ge2N2O or Sn2N2O (JP 3918311).
Among others, silicon oxide is represented by SiOx wherein x is slightly greater than the theoretical value of 1 due to oxide coating, and is found on X-ray diffractometry analysis to have the structure that amorphous silicon ranging from several to several tens of nanometers is finely dispersed in silica. For this reason, silicon oxide has a battery capacity which is greater than that of currently used carbon by a factor of 5 or 6 on a weight basis, and a less volume expansion and hence, relatively good cycle performance. The silicon oxide is believed fully useful in the application to portable equipment such as mobile phones, laptop computers and tablets. However, when the automotive application is considered, the silicon oxide must overcome the problems of insufficient cycle performance and a high cost.